<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <style>
        p {
            font-size: 3em;/* 1. use "em" to replace "px", 1em = 16px*/
        }
    </style>
</head>
<!--2. Set percentage to control the font-size-->
<body style="font-size: 70%">
<p>First</p>
<h1>Second</h1>
</body>
</html>